Bonding of oriented-fiber nonwoven fibrous webs often requires an undesirable compromise in processing steps or product features. For example, when collected webs of oriented fibers such as meltspun or spunbond fibers are bonded (e.g., to consolidate the web, increase its strength, or otherwise modify web properties), a bonding fiber or other bonding material is typically included in the webs in addition to the meltspun or spunbond fibers. Alternatively or in addition, the web is subjected to heat and pressure in a point-bonding or area-wide calendering operation. Such steps are required because the meltspun or spunbond fibers themselves generally are highly drawn to increase fiber strength, leaving the fibers with limited capacity to participate in fiber bonding.
But addition of bonding fibers or other bonding material increases the cost of the web, makes the manufacturing operation more complex, and introduces extraneous ingredients into the webs. And heat and pressure changes the properties of the web, e.g., making the web more paperlike, stiff, or brittle.
Bonding between spunbond fibers, even when obtained with the heat and pressure of point-bonding or calendering, also tends to be of lower strength than desired: the bond strength between spunbond fibers is typically less than the bond strength between fibers that have a less-ordered morphology than spunbond fibers have; see the recent publication, Structure and properties of polypropylene fibers during thermal bonding, Subhash Chand et al, (Thermochimica Acta 367-368 (2001) 155-160).
While the art has recognized the deficiencies involved in bonding of oriented-fiber webs, no satisfactory solution is known to exist. U.S. Pat. No. 3,322,607 describes one effort at improvement, suggesting among other bonding techniques that fibers be prepared having mixed-orientation fibers, in which some segments of the fibers have a lower orientation and thereby a lower softening temperature such that they function as binder filaments. As illustrated in Example XII of this patent (see also column 8, lines 9-52), such mixed-orientation fibers are prepared by leading extruded filaments to a heated feed roll and engaging the filaments on the roll for some time while the roll rotates. Low-orientation segments are said to result from such contact and to provide bondability in the webs. (See also U.S. Pat. No. 4,086,381, for example, at column 5, line 59 et seq, for a similar teaching.)
But the low-orientation bonding segments of the fibers in U.S. Pat. No. 3,322,607 are also of greater diameter than other segments of higher orientation (col. 17, 11. 21-25). The result is that increased heat is needed to soften the low-orientation segments to bond the web. Also, the whole fiber-forming process is operated at a rather low speed, thereby decreasing efficiency. And according to the patent (col. 8, 11. 22-25 and 60-63) the bonding of the low-orientation segments is apparently insufficient for adequate bonding, with the result that bonding conditions are selected to provide some bonding of the high-orientation segments or fibers in addition to the low-orientation segments.
Improved bonding methods are needed, and it would be desirable if these methods could provide autogenous bonding (defined herein as bonding between fibers at an elevated temperature as obtained in an oven or with a through-air bonder—also known as a hot-air knife—without application of solid contact pressure such as in point-bonding or calendering), and preferably with no added binding fiber or other bonding material. The high level of drawing of meltspun or spunbond fibers limits their capacity for autogenous bonding. Instead of autogenous bonding, most single-component meltspun or spunbond fibrous webs are bonded by use of heat and pressure, e.g., point-bonding or a more area-wide application of heat and calendering pressure; and even the heat-and-pressure processes are typically accompanied by use of bonding fibers or other bonding material in the web.